Photosynthesis 2.9, 8.3 Flashcards
What is the general equation of photosynthesis?
6 CO2 + 6 H2O → C6H12O6 + 6 O2
What are the ranges of wavelengths in each spectrum?
- visible light are between 400 and 700 nanometers
- longer wavelengths (infrared) = low energy
- shorter wavelengths (UV) = high energy
- plants absorb light because it is most abundant in nature
Chlorophyll
- pigment
- absorbs red and blue light
- doesn’t absorb green → reflected
- used by plants to harvest light energy
Thin layer chromatography
- Blending plant tissue and gradually adding propanone
- Putting pigment onto plastic strip
- Marking the level of pigment on the tube
- Adding running solvent.
- After 5 minutes, taking plastic strip out and measuring the distance the solvent did and the pigments did. —> Rf is the fraction of these
From this value we can recognise different pigments
Chloroplast
Outer membrane — separates from the rest (specialised compartment)
Chloroplast envelope — created by double membrane
Thylakoid — internal membrane (intense green)
Fluid filled spaces in thylakoid
Stroma — colourless fluid around thylakoid. Contains many enzymes
Grand — stacks of thylakoid
Starch grains and lipid droplets — if plant is photosynthesising quickly
Photoactivation
- photosystems = chlorophyll and other pigments together to harvest light
- Photosystem I and II
- centre → energy absorbed by chlorophyll is moved to special chlorophyll molecules
- energy from light absorbed → e- excited
- chlorophyll molecule photoactivated
- transfers e- to e- acceptor
- Photosystem II → e- acceptor = plastoquinone takes 2e-
- plastoquinone moves to the other complex
- hydrophobic → inside the membrane
- plastoquinone moves to the other complex
2 photons of light → plastoquinone
1 chlorophyll → 2e-
for 2 plastoquinones: 4e- from 2 chlorophyll molecules
Photolysis
- in thylakoid fluid
- photolysis = oxidation of water
- for chlorophyll which lost 4e-
2 H2O —> O2 + 4 H+ + 4 e-
- oxygen is a waste product → diffuses away
Where does light-dependent reaction occur?
- in thylakoid membrane (third membrane)
The electron transport chain
In photosynthesis
- photophosphorylation = producing ATP from light
- in thylakoid
- Photosystem II → electron transport chain → Photosystem I
- H+ move through ATP synthase
- plastoquinone carries electrons to the chain of electron carriers from Photosystem II
Proton gradient
In photosynthesis
- electrons pass through electron transport chain
- protons into thylakoid space → gradient created
- photolysis also adds to gradient
Chemiosmosis (photosynthesis)
- H+ down the gradient through ATP synthase
- energy released → phosphorylates ADP
- electrons are finally accepted by plastocyanin in the thylakoid fluid
Reduction of NADP
In photosynthesis
- Photosystem II passes electrons to plastocyanin
- re-excited in Photosystem I (photoactivation)
- passed to ferredoxin → 2 e- to reduce NADP → NADPH
- 12 H+ pumped into thylakoid space
- around 3 ATP molecules
- 8 photons absorbed
- supply of NADP is out
- electron moves back to chain of electron carriers
- protons pumped and energy made = cyclic photophosphorylation
- electron moves back to chain of electron carriers
Carbon fixation (step 1 of Calvin cycle)
- CO2 used to create carbohydrates
- ribulose bisphosphate (RuBP) 5 C molecule + CO2 → GP (glycerate-3-phosphate)
- catalysed by rubisco = ribulose bisphosphate carboxylase
- a lot of rubisco in stroma to maximise carbon fixation
3 RuBP + 3 CO2 —> 6 GP
Reduction of GP (glycerate-3-phosphate)
- CO2 + RuBP → GP (catalysed by rubisco)
- ratio of H:O wrong
- hydrogen added to GP (reduction)
- 6 ATP —> 6 ADP + 6P (energy) and 6 NADPH —> 6 NADP+ + 6 H+ (reducing agent)
- ATP and NADPH from light-dependent reactions
- 6 GP → 6 TP (or G3P) - triose phosphate
Regeneration of RuBP
In Calvin cycle
- RuBP must be regenerated for new cycle
- 5 TP → 3 RuBP
- only 1 TP left (after 3 cycles)
- energy 3 ATP → 3 ADP + 3 P
Calvin’s lollipop experiment
- 12C was exchanged with 14C (radioactive)
- photosynthesis stopped at different time intervals t
- radioactive carbon compounds found
- showed the steps of Calvin cycle
Photosynthesis impact on the Earth
- oxygen as by-product
- Great Oxidation Event = rise in oxygen concentration in atmosphere
- reduction in methane and CO2 concentration → greenhouse effect reduced
- glaciation
- reduction in methane and CO2 concentration → greenhouse effect reduced
- iron precipitated in water (oxidised)
- rock formation
- multicellular organisms evolved = significant rise in O2 levels
What are the limiting factors of photosynthesis?
Temperature, light intensity, carbon dioxide concentration
When below optimal level they slow down the reaction. Factor furthest from the optimum is the limiting one.
